Devices with predetermined reverse-conduction thresholds are used in various applications in order to limit the maximum voltage which can be applied between two nodes of a circuit. A typical example is one or more zener diodes connected in series (such as a zener chain). As is known, in a zener diode, when the reverse voltage applied between the anode and the cathode exceeds a certain threshold value (Vz) the diode becomes conductive in reverse with a variable current and a substantially constant voltage. This reverse threshold voltage, known as the zener voltage, varies from a few volts to a few tens or hundreds of volts. A device with a predetermined reverse-conduction threshold can also be formed with the use of a bipolar transistor with an open base taken between the emitter and the collector, making use of the reversible breakdown, or "punch-through," effect of the base, or of a diode the conduction characteristic of which, although not based on the zener effect, is similar to that of a zener diode, that is, a predetermined, non-destructive breakdown threshold voltage like most diodes with avalanche reverse-breakdown junctions.
In order to limit the maximum voltage which can be applied between two nodes of a circuit, a chain of zener diodes having a total reverse threshold voltage equal to the voltage which is not be to exceeded is inserted between them. As long as the voltage applied between the two nodes remains below this value, the zener diodes behave as an open circuit. When the voltage exceeds the reverse threshold voltage of the zener chain, however, the zener diodes become conductive, maintaining a substantially constant voltage at the terminals of the zener chain.
These devices with predetermined reverse-conduction thresholds are often used in power applications in which the voltage which has to be maintained at the two nodes of the circuit reaches high values, typically greater than 50V (for example, 400V). An example of these applications is electronic ignitions implemented by integrated circuits. Such a circuit comprises a power transistor used as a switch for the switched supply of a load with currents which may be very high. When the load of the transistor is inductive, the opening of the switch, that is, the instantaneous change from the conductive state to the cut-off state of the transistor, causes a transitory surge voltage condition between the collector and emitter terminals of the power transistor. This surge voltage may also be several volts higher than the supply voltage of the transistor and may reach values and durations such as to exceed the safety limits set by the structural characteristics of the transistor. In these applications, a protection device implemented by a zener diode or a chain of zener diodes is commonly inserted between the collector and the base of the power transistor. When the collector-emitter voltage increases as a result of the change from the conductive to the cut-off condition, and until the reverse-conduction value of the zener chain is reached, a current is injected into the base of the transistor. The transistor therefore becomes conductive again and discharges to itself the energy stored by the inductive load.
In a high-voltage device with a predetermined reverse-conduction threshold, the value of the reverse threshold voltage is highly dependent upon the temperature. For example, in the case of a high-voltage zener diode, the reverse threshold voltage varies with temperature because of the high thermal drift of these components. In particular, the reverse threshold voltage increases as the temperature increases so that the temperature coefficient of a high-voltage zener diode, that is, the ratio between the reverse threshold-voltage variation and the temperature variation (dVz/dT) is positive. For example, a zener diode may demonstrate a thermal drift of 0.1% per .degree. C. (1 mV/V.degree. C.); this means that, for a 400V chain of zener diodes there is a variation of 400 mV/.degree. C. This phenomenon is the source of inaccuracies which may be unacceptable in many applications.
A technique known in the art for the thermal compensation of a zener chain is that of the use of one or more diodes placed in series with the zener chain so that they are polarized directly during the reverse-conduction stage of the zener diodes. The threshold voltage of a diode (Vbe) in fact decreases as the temperature increases, so that the temperature coefficient of a diode (dvbe/dT) is negative; a diode having a threshold voltage of 0.7V typically has a variation of -2 mv/.degree. C. It is therefore possible, by a suitable circuit design, to compensate for the increase of the reverse threshold voltage of the zener diodes with temperature by a corresponding equal variation, with the opposite sign, of the threshold voltage of the compensation diodes.
However, this solution cannot easily be used for high-voltage devices with predetermined conduction thresholds produced in integrated form, because of the very high number of compensation diodes required. For example, a reverse-conduction threshold device comprising a chain of high-voltage zener diodes and a series of compensation diodes which has to maintain a constant voltage of 400V will be considered. If the reverse threshold voltage of the zener chain is indicated Vzc and the number of compensation diodes connected in series therewith is indicated K: EQU Vzc+K.multidot.Vbe=400V
In order to achieve full temperature compensation of the device, the variation of the voltage at its terminals with variations in temperature must be zero, that is: ##EQU1##
Upon the assumption that Vbe=700 mV, dVbe/dT=-2 mv/.degree. C., dVzc/dT=1 mV.multidot.Vzc: EQU 1.multidot.10.sup.-3 .multidot.Vzc-2.multidot.10.sup.-3 .multidot.K=0
from which, by substituting Vzc=400-K.multidot.0.7: EQU 1.multidot.10.sup.-3 .multidot.400-1.multidot.10.sup.-3 .multidot.K.multidot.0.7-2.multidot.10.sup.-3 .multidot.K=0 EQU 2.7.multidot.10.sup.-3 .multidot.K=1.multidot.10.sup.-3 .multidot.400
and hence: EQU K=148
The value of the reverse threshold voltage of the zener chain is consequently: EQU Vz=400-148.multidot.0.7=296.4V
Thus, in a device with a reverse threshold voltage of 400V, almost 100V has to be attributed to the compensation diodes by the insertion of 148 diodes in series with the zener chain.
This solution involves the occupation of an excessive area of the chip of the monolithic integrated circuit. Moreover, it requires considerable care in production in order to prevent structural current leakages between the various integrated diodes; this results in the occupation of further area. Moreover, each compensation diode has to be of a size such as to have an adequate area and dynamic resistance for the working current of the zener chain; for example, in the case of the electronic-ignition integrated circuit described above, the thermal-compensation diodes have to be able to withstand the base current necessary to switch the power transistor on again.